add PST to main SECURITY_CONTACTS as formality
add PST to main SECURITY_CONTACTS as formality

Add the Product Security Team as the security contacts for the main
repository and they can use the OWNERS files in each subsystem/dir to find
the correct owners.

Signed-off-by: Jess Frazelle <acidburn@microsoft.com>

Kubernetes-commit: ac015892e4e2ab641a7761ab79f8cf986e1fa266

kubernetes/sample-apiserver,kubernetes-nightly/sample-apiserver,kubernetes-nightly/sample-apiserver,kubernetes/sample-apiserver